Electrostatic discharge (ESD) is a serious problem confronting circuit designers. Electrostatic voltages can build up from a variety of environmental sources and can reach thousands or even tens of thousands of volts. During discharge, high transient currents can damage equipment by generating high temperatures that can melt circuit components.
An ESD protection circuit is typically added to integrated circuits (ICs) at bond pads. The pads are the connections to outside circuits, for all electric power supplies, electric grounds, and electronic signals. An ESD protection circuit has two states: normal operation mode and ESD mode. When an IC is in the normal operation mode, the ESD protection circuit appears invisible to the IC by blocking current through itself. In the ESD mode, the ESD protection circuit serves its purpose of protecting the IC by conducting an electrostatic charge quickly to VSS, or ground, before damage can occur to the internal circuit.
ESD protection is an important consideration for the design and manufacture of radio frequency identification (RFID) tags. An RFID tag, also known as an RFID transponder, is a device used to communicate with an RFID reader (also known as interrogator) in an RFID system. Tags can be active, semi-active, or passive. Active tags carry power supplies, which power RFID circuitry and communicate with the reader. The power supply for a semi-active tag is used to power the electronics but not to communicate with the reader. A passive tag relies entirely on the reader to provide sufficient energy to operate the circuitry and communicate with the reader. In a passive RFID tag, the input pin for an antenna is connected to a metal-insulator-metal (MIM) capacitor followed by diodes for signal detection. Static electricity is commonly generated during the manufacturing of RFID labels, and the MIM structure is at risk of being damaged during the process of assembling the antenna. It is therefore important to provide an ESD bypass path to prevent ESD events during antenna assembly and testing.
Conventional ESD protection circuits may alter the input impedance of RFID circuits at a frequency of interest, resulting in significantly degraded power transfer efficiency and thereby placing practical limits on the extent of ESD protection that is possible in an RFID tag using conventional techniques. An RFID ESD protection device should provide a low impedance path to route ESD current around an underlying core RFID circuit during ESD events while being transparent to the core circuit during normal RF operation. The input power from an RFID antenna may be up to 0 dBm, corresponding to an amplitude of about 7 V.
Conventional ESD protection circuits are unsuitable for large signal (high swing) RFID operation due to a low trigger voltage that disadvantageously results in ESD protection turn-on during normal RF operation. Conventional ESD protection circuits are also susceptible to latching up (remaining in an ON state) due to a low holding voltage that does not allow ESD protection turn-off.
It is desirable to protect circuitry in an RFID tag without exhibiting the latch-up problem and without interfering with normal RFID operation.